The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
Japanese Patent Laid Open No. 58-88252 discloses an electronic control system for controlling the line pressure in accordance with transmission ratio and engine torque. The transmission ratio is determined by the ratio of the speed of the driven pulley to the speed of the drive pulley. When the vehicle is at a stop, both pulleys stop because of disengagement of a clutch. Since input signals based on the speeds of both pulleys disappear, it is impossible to control the line pressure dependent on the transmission ratio.